Fire alarm device

ABSTRACT

Fire alarm devices wherein a detector element such as an ionization chamber includes a field-effect transistor for amplifying an electrical signal produced by the detector and a cover to protect the gate electrode of the field-effect transistor including switch means operable on removal of the cover to short circuit and the gate electrode to another electrode of the transistor and to remove the short circuit on proper replacement of the cover.

United States Patent Andreas Scheidweiler;

Max Kuhn, both of Stafa. Switzerland [21] Appl. No. 749,166

[22] Filed July 31, 1968 [45] Patented June 29, 1971 [72] Inventors [73]Assignee Cerberus AG Mannedorl', Switzerland [32] Priority Aug. 9, 1967[3 3] Switzerland [54] FIRE ALARM DEVICE 9 Claims, 5 Drawing Figs.

sz1u.s.c1 317/16,

200/6162, 317/33, 317/52 51 1111.c1...........1.... 1102115/12 soFieldofSearch 335/186,

202; l74/l7.4; 317/9, 235/2l.1,16, 33, 52; ZOO/50.1, 61.62, 61.71

[56] References Cited UNITED STATES PATENTS 2.756.299 7/1956 Sunko etal.. .1 ZOO/61.62 3,222,473 12/1965 Delhase..... ZOO/61.62 3,413.49712/1968 Atalla 317/33 X Primary Examiner-James D. TrammellAttorney-Werner W. Kleeman ABSTRACT: Fire alarm devices wherein adetector element such as an ionization chamber includes a field-effecttransistor for amplifying an electrical signal produced by the detectorand a cover to protect the gate electrode of the field-effect transistorincluding switch means operable on removal of the cover to short circuitand the gate electrode to another electrode of the transistor and toremove the short circuit on proper replacement of the cover.

FIRE ALARM DEVICE I BACKGROUND or THE INVENTION This invention relatesto fire alarm devices of the type containing at least one detectorelement sensitive to a phenomenon resulting from combustion, afield-effect transistor for amplifying an electrical signal produced bythe detector element, connections for an electrical circuit for givingan alarm and a cover which protects the gate electrode of thefield-effect transistor and the circuit components connected with itagain external contact.

The desire for tire alarm devices which can be operated with a lowoperating voltage has led to the increasing transistorization of thealarm devices, leading to less technical skill for installation, readiercompliance with safety specifications and lower power consumption. It istrue that for a long time the main difficulty was to replace thecold-cathode gasdischarge tubes or equivalent devices and circuits whichhad to be employed whenever a very high impedance amplifier element wasneeded to amplify the electrical signals appearing across the detectorelement. This is particularly the case in ionization fire alarm devicesemploying one or more ionization chambers, in which the amplifierelement must possess an input resistance of the order of magnitude of atleast ohms. An element suitable for this purpose could be found only inthe field-effect transistor, the inclusion of which in mass-producedfire alarm devices has, however, led to certain difficulties. Thebreakdown voltage between the gate electrode and one of the other twoelectrodes (source and drain electrode) of the field-effect transistoramounts to only some 50 v.; its input capacitance may become charged toseveral hundred volts by a touch from the naked hand on the gateelectrode. Since this charge cannot flow away, a breakdown occurs whichin contrast to conventional transistors with diode characteristics,effects irreparable damage. It is, of course, true that in operation afield-effect transistor may be covered by a suitable cover, possibly thecasing of the alarm device, to protect against any direct contact withthe gate electrode or with any parts of the alarm device connected withthe gate electrode. This covering must however be removed for purposesof cleaning or repair, so that it has been found by experience that therisk of damage to the field-effect transistor is very high.

SUMMARY OF THE INVENTION Accordingly, it is a primary object of theinvention to provide an improved protective device for fire alarmdevices of the kind described which will protect the field-effecttransistor from deleterious charging during maintenance operations.

Another object of this invention is to provide a switch means operableon removal of a cover over the field-effect transistor to short circuitthe gate electrode to another electrode of the transistor.

Yet a further object of the invention is the use of either a resilientlybiased switch member which is movable by a guide element on displacementof the cover or a reed switch and a permanent magnet with aferromagnetic member interposable therebetween on proper positioning ofthe cover to prevent flux from the magnet from actuating the switch orwherein the member of ferromagnetic material forms on additional permanent magnet which neutralizes the field of the first permanent magnet.

Other and further objects will be particularly pointed out or will beobvious from the following detailed description.

According to the invention there is provided a fire alarm deviceincluding a detector element responsive to a phenomenon resulting fromcombustion to generate an electrical signal, a field-effect transistorarranged to amplify the signal and to supply an amplified signal to analarm circuit and cover means normally preventing external contact withthe to short circuit said gate electrode to another electrode of saidtransistor and operable in response to the correct placement of saidcover means to remove the short circuit.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be furtherdescribed with reference to the accompanying drawings in which:

FIG. 1 shows a cross section through a fire alarm device incorporatingone embodiment of the invention;

FIG. 2a shows in cross section a modification of that part of the deviceof FIG. 1 lying to the left of the line A-A;

FIG. 2b shows a plan view of the modified embodiment shown in FIG. 2a,partly in cross section taken along line 8-8;

FIG. 3 shows in cross section another modification or the apparatus ofFIG. 1; and

FIG. 4 shows a circuit diagram of a fire alarm device incorporating theinventive concepts.

Like reference characters refer to like parts throughout the severalviews of the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS S The fire alarm devices towhich the illustrations relate are known as ionization fire alarms inwhich the detector element sensitive to a phenomenon resulting from afire consists of an ionization chamber including a radioactivesubstance. As shown in FIG. 4 the measurement ionization chamber 10 isconnected in series with a reference ionization chamber 11 and connectedby way of supply leads or conductors l and 3 to a central station (notshown). The common connecting point or junction 50 (FIG. 4) of the twoionization chambers 10 and 11 is taken to the gate electrode 14 of afield-effect transistor 12 which amplifies the alterations of voltageappearing at this point of the circuit and delivers at its sourceelectrode 51 a signal which is applied by way of an alarm terminal 2 toa known alarm circuit (not shown). In FIG. 4 the source electrode 51 oftransistor 12 is connected by way of a resistor 9 to supply lead 3. Inaccordance with the invention there is connected between the drainelectrode 13 and the gate electrode 14 a short-circuiting switch 15which is open during normal operation, that is with the cover of thealarm device closed. The short-circuiting switch may equally well bearranged between the gate electrode and the source electrode, as shownin broken lines at 15'.

FIG. 1 shows the greatly simplified construction of an ionization firealarm device in accordance with FIG. 4. Into a plate 16 of insulatingceramic material there is introduced an electrode 17 common to ameasuring ionization chamber 10 and a reference ionization chamber 11.Two radioactive sources 18 and 19 ionize the air molecules present inthe chambers 10 and II respectively. The reference ionization chamber 11is provided with an electrically conductive housing 20 which rests uponan insulating base 30 and is effectively airtight; the external air haspractically free entry into the measuring chamber 10, which is boundedby a housing 21 provided with perforations 22. The housing 20 of thereference ionization chamber 11 also forms the second electrode of thatchamber, and the housing 21 of the measuring ionization chamber 10 formsthe second electrode thereof. The housing 21 of the measuring ionizationchamber 10 is push or screw mounted on to a baseplate 23 and may beremoved for cleaning or repair purposes; the casing or housing of themeasuring chamber 10 is connected by way of a contact member 29 withsupply lead 3 and is usually grounded. During operation the measuringchamber housing 21 also serves as a cover which protects the individualparts of the alarm device, and especially those elements which are inconnection with the gate electrode 14 of the field-effect transistorfrom direct contact.

In order that the field-effect transistor 12 shall also remain protectedagainst excessive charging even with the cover removed, a switch 15 isprovided, as already explained with reference to FIG. 4 which, by adevice described below in more detail, is automatically closed uponremoval of the measuring chamber housing 21, thus short circuiting thegate elec trode 14 of the field-effect transistor 12 to one of the otherelectrodes, as shown for example, to the drain electrode 13. In theembodiment of H6. 1 the switch 15 consists of a movable spring contactwhich, if no external 'force is applied, is resiliently biased intoengagement with a fixed contact 24. Fixedly connected to themeasuringchamber housing 21 is a disc 25 of insulating material with aprotruding cylindrical flange 26. A guide member 27, aligned with flangeor ring 26, restrains pin 28 which, when the chamber housing .21 iscorrectly positioned, moves spring contact 15 from its rest position andaway from the fixed contact 24. The short circuit between gate electrode14 and drain electrode 13 by way of reference-chamber housing 20, springcontact 15, fixed contact 24 and electrode 17 is thus broken and thefield-effect transistor is made operative. On the other hand, on removalof the measuring-chamber housing 21, the pin 28 is urged upward(relative 'to the showing in the drawing) and gate electrode 14 of thefield-effect transistor 12 is connected to the drain electrode 13.

FIG. 2a which represents a modification of that part of the fire alarmdevice of FIG. 1 which lies to the left of line A-A shows anotherarrangement of switch 15 and its actuating means. FlG. 2b is a partialsectional plan view of the device of FIG. 2a, taken along the line B-B.Switch 15 which in its rest position short circuits the gate and drainelectrodes of the field-effect transistor, employs here a reed switch 31having its contacts enclosed in a protective gas atmosphere. Since, incontradistinction to the spring contact of FIG. 1, the contacts of reedswitch 31 are normally open, a permanent magnet 32 mounted on a base 33is provided, which closes the contacts of reed switch 31 when thehousing 21 of the measuring chamber is removed. With the cover 21correctly placed there is interposed between the magnet 32 and theswitch 31 a member of ferromagnetic material which prevents theflux fromthe permanent magnet from actuating the switch. Thus, more specifically,there is fixedly secured to the disc 25 of the measuring chamber a ring34, which in this case consists of a high permeability ferromagneticmaterial and which, when the measuring chamber housing 21 is in positionfor operation, is positioned between the permanent magnet 32 and thereed switch 31 to screen the flux of the magnet from the switch, thuspreventing'the magnet from actuating the reed switch. One contact ofreed switch 31 is connected by way of a lead 35 with electrode 17 andthe other by way of a lead 36 with the housing 20 of the referencechamber. A cover 37 is provided to protect the components of the alarmdevice against the entry of external air, dust, etc.

Finally, FIG. 3 shows a modification of the alarm device of FIG. 20,using a reed switch, in which ring 32 fitted to disc 25 now consists ofa further permanent magnet, which when the measuring-chamber housing iscorrectly positioned for operation is positioned so as to neutralize thefirst permanent magnet and to prevent actuation of the reed switch 31 sothat the contacts of reed switch 31 remain open. The remainingreferences correspond with those of FIG. 1.

The invention has been particularly described only with reference toionization fire alarm devices, but it will readily be appreciated thatthe inclusion of a short circuiting switch and a device actuated by thecover which determines the setting of the switch can advantageously beutilized in any kind of fire alarm device in which a field-effecttransistor is employed as an amplifying device. Thus in FIG. 4 thedetector element 10 might be replaced by a temperature-dependentresistor or a photocell, and the reference element 11 by a resistor orby some other suitable bridging element, as will be apparentto thoseskilled in the art.

Having now described in detail the instant inventive concepts,

' What we claim is:

1 In a fire alarm device including a detector element responsive to aphenomenon resulting from combustion to generate an electrical signal, afield-effect transistor having gate, source and drain electrodes,arranged to amplify the signal and to supply an amplified signal to analarm circuit, and cover means normally preventing external contact withthe gate electrode of said field-effect transistor, the improve mentwhich comprises switch means operable in response to the displacement ofsaid cover means to short circuit said gate electrode to anotherelectrode of said transistor, to thereby protect said transistor againstdamage from static charges, and said switch means being operable inresponse to the correct placement of said cover means to remove theshort circuit.

' 2. In a fire alarm device as defined in claim 1, wherein said switchmeans includes a movable contact resiliently biased into engagement witha fixed contact, and actuating means operated on correct positioning ofsaid cover means to force said movable contact away from said fixedcontact.

3. in a fire alarm device as defined in claim 2, wherein said actuatingmeans comprises a pin freely movable in a guide, a member secured tosaid cover means which engages said pin when said cover means tocorrectly positioned, said pin engaging said movable switch contact toforce it away from said fixed contact.

4. In a fire alarm device including a detector element responsive to aphenomenon resulting from combustion to generate an electrical signal, afield-effect transistor having -gate, source and drain electrodes,arranged to amplify the signal and to supply an amplified signal to analarm circuit, and cover means normally preventing external contact withthe gate electrode of said field-effect transistor, the improvementwhich comprises switch means operable in response to the displacement ofsaid cover means to short circuit said gate electrode to anotherelectrode of said transistor and operable in response to the correctplacement of said cover means to remove the short circuit, said switchmeans including a reed switch having resiliently biased contact members,a permanent magnet whose magnetic field extends between said permanentmagnet and said reed switch in order to urge together said contactmembers against their resilient bias when the cover means is removed,and a member of ferromagnetic material which prevents the flux from thepermanent magnet from actuating said reed switch when the cover means iscorrectly positioned.

5. In a fire alarm device as defined in claim 4, wherein said member offerromagnetic material is of high permeability and screens the magneticflux of the permanent magnet from said reed switch.

6. In a fire alarm device as defined in claim 4, wherein said member offerromagnetic material forms a permanent magnet which neutralizes thefield of the first said permanent magnet to prevent actuation of saidreed switch.

7. In a fire alarm device including a detector element responsive to aphenomenon resulting from combustion to generate an electrical signal, afield-effect transistor having gate, source and drain electrodes,arranged to amplify the signal and to supply an amplified signal to analarm circuit, and cover means normally preventing external contact withthe gate electrode of said field-effect transistor, the improvementwhich comprises switch means operable in response to the displacement ofsaid cover means to short circuit said gate electrode to anotherelectrode of said transistor and operable in response to the correctplacement of said cover means to remove the short circuit, said switchmeans being connected between the gate and the source electrodes of saidfield-effect transistor.

8. In a fire alarm device including a detector element responsive to aphenomenon resulting from combustion to generate an electrical signal, afield-effect transistor having gate, source and drain electrodes,arranged to amplify the signal and to supply an amplified signal to analarm circuit, and cover means normally preventing external contact withthe gate electrode of said field-effect transistor, the improvementwhich comprises switch means operable in response to the displacement ofsaid cover means to short circuit said gate electrode to anotherelectrode of said transistor and operable in response to the correctplacement of said cover means to remove the short circuit, said switchmeans being connected between the gate and drain electrodes of thefield-effect transistor.

9. In a fire alarm device including a detector element responsive to aphenomenon resulting from combustion to generate an electrical signal, afield-effect transistor having gate, source and drain electrodes,arranged to amplify the signal and to supply an amplified signal toanalarm circuit,

1. In a fire alarm device including a detector element responsive to aphenomenon resulting from combustion to generate an electrical signal, afield-effect transistor having gate, source and drain electrodes,arranged to amplify the signal and to supply an amplified signal to analarm circuit, and cover means normally preventing external contact withthe gate electrode of said field-effect transistor, the improvementwhich comprises switch means operable in response to the displacement ofsaid cover means to short circuit said gate electrode to anotherelectrode of said transistor, to thereby protect said transistor againstdamage from static charges, and said switch means being operable inresponse to the correct placement of said cover means to remove theshort circuit.
 2. In a fire alarm device as defined in claim 1, whereinsaid switch means includes a movable contact resiliently biased intoengagement with a fixed contact, and actuating means operated on correctpositioning of said cover means to force said movable contact away fromsaid fixed contact.
 3. In a fire alarm device as defined in claim 2,wherein said actuating means comprises a pin freely movable in a guide,a member secured to said cover means which engages said pin when saidcover means to correctly positioned, said pin engaging said movableswitch contact to force it away from said fixed contact.
 4. In a firealarm device including a detector element responsive to a phenomenonresulting from combustion to generate an electrical signal, afield-effect transistor having gate, source and drain electrodes,arranged to amplify the signal and to supply an amplified signal to analarm circuit, and cover means normally preventing external contact withthe gate electrode of said field-effect transistor, the improvementwhich comprises switch means operable in response to the displacement ofsaid cover means to short circuit said gate electrode to anotherelectrode of said transistor and operable in response to the correctplacement of said cover means to remove the short circuit, said switchmeans including a reed switch having resiliently biased contact members,a permanent magnet whose magnetic field extends between said permanentmagnet and said reed switch in order to urge together said contactmembers against their resilient bias when the cover means is removed,and a member of ferromagnetic material which prevents the flux from thepermanent magnet from actuating said reed switch when the cover means iscorrectly positioned.
 5. In a fire alarm device as defined in claim 4,wherein said member of ferromagnetic material is of high permeabilityand screens the magnetic flux of the permanent magnet from said reedswitch.
 6. In a fire alarm device as defined in claim 4, wherein saidmember of ferromagnetic material forms a permanent magnet whichneutralizes the field of the first said peRmanent magnet to preventactuation of said reed switch.
 7. In a fire alarm device including adetector element responsive to a phenomenon resulting from combustion togenerate an electrical signal, a field-effect transistor having gate,source and drain electrodes, arranged to amplify the signal and tosupply an amplified signal to an alarm circuit, and cover means normallypreventing external contact with the gate electrode of said field-effecttransistor, the improvement which comprises switch means operable inresponse to the displacement of said cover means to short circuit saidgate electrode to another electrode of said transistor and operable inresponse to the correct placement of said cover means to remove theshort circuit, said switch means being connected between the gate andthe source electrodes of said field-effect transistor.
 8. In a firealarm device including a detector element responsive to a phenomenonresulting from combustion to generate an electrical signal, afield-effect transistor having gate, source and drain electrodes,arranged to amplify the signal and to supply an amplified signal to analarm circuit, and cover means normally preventing external contact withthe gate electrode of said field-effect transistor, the improvementwhich comprises switch means operable in response to the displacement ofsaid cover means to short circuit said gate electrode to anotherelectrode of said transistor and operable in response to the correctplacement of said cover means to remove the short circuit, said switchmeans being connected between the gate and drain electrodes of thefield-effect transistor.
 9. In a fire alarm device including a detectorelement responsive to a phenomenon resulting from combustion to generatean electrical signal, a field-effect transistor having gate, source anddrain electrodes, arranged to amplify the signal and to supply anamplified signal to an alarm circuit, and cover means normallypreventing external contact with the gate electrode of said field-effecttransistor, the improvement which comprises switch means operable inresponse to the displacement of said cover means to short circuit saidgate electrode to another electrode of said transistor and operable inresponse to the correct placement of said cover means to remove theshort circuit, said detector element being an ionization chamber.